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Ab initio study of the chloromethane dication fragmentation
and Ion Processes
ELS EVI E R
International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Ab initio study of the chloromethane dication fragmentation Denis Duflot*, Jean-Michel Robbe, Jean-Pierre Flament Laboratoire de Dynamique Moldculaire et Photonique, URA CNRS 779. Centre d'Etudes et de Recherches Lasers et Applications, Universit( des Sciences et Technologies de Lille, UFR de Physique, 59655 Villeneuve d'Ascq Cedex, France
Received 7 May 1997; revised 20 June 1997; accepted 6 July 1997
Abstract The chloromethane dication dissociation pathways have been studied using the complete active space self-consistent field (CASSCF) method followed by a multi-reference perturbative configuration interaction (CI). The vertical double ionization energy is calculated to be 31.64 eV for the first singlet state and 30.69 eV for the first triplet state, the latter value being close to the experimental determination of 31.5 -+ 0.5 eV. For most two-body dissociation processes, the appearance thresholds of the ion pairs observed in coincidence experiments are in good agreement with the results of the calculations. In the case of the CCI+/H~ pair, the dissociation is indirect and comes from the ionization of a Rydberg excited state of the CH3CI ÷ monocation. For three-body processes, the uncertainties of the available experimental results and the large number of possible dissociation channels do not allow identification uniquely of the involved mechanisms in most cases. © 1997 Elsevier Science B.V. K e y w o r d s : Configuration interaction; Chloromethane; Dications; Fragmentation
1. Introduction
In a previous paper [1], we used large scale ab initio techniques to study the stability of the two lowest states of the CH3C12+ dication. We found that the 3A 2 and IA' states were stable, contrary to the calculations performed at Hartree-Fock level with a 6-31G* basis set by Yates and coworkers [2]. While t h e 3A 2 state keeps the C3v character of the neutral species, the ~A' state is distorted because of an important Jahn-Teller effect [3,4]. In the present work, we present a theoretical interpretation of the available experimental data on the fragmentation of CH3C12÷. The CH3C12+ fragmentation has been the * Tel: 00 33 3 20 43 66 96; Fax: 00 33 3 20 43 40 85; E-mail: duflot @lsm350.univ-lille I .fr
subject of only a few experimental studies. Some years ago, Monce and co-workers [5,6] observed the CH~ and C1+ fragments after bombarding neutral CH3CI with 1 MeV H +, He + and O + projectiles. The authors deduced from coincidence measurements that these fragments came from the dissociation of a CH3C12+ precursor. More recently, Ruhl and co-workers [7] used the PEPIPICO method (photoelectron-photoionphotoion coincidence) [8] to obtain the relative abundance and the kinetic energy release (KER) of each ion pair at an excitation energy of 40.8 eV. The PEPICO method (photoeelectronphotoion coincidence) combined with synchrotron radiation has also been used to study the excitation of the Cl(2p) shell [9,10] and to obtain the appearance thresholds of the fragmentation products at low energies [11]. The main
0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 1 1 7 6 ( 9 7 ) 0 0 1 2 3 - 7
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D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Table 1 S u m m a r y of experimental results on the fragmentation of the CH3C12+ dication Products
Neutral
Threshold a
Intensity b
KER b
(eV)
(% pairs at 40.8 eV)
(40.8 eV)
Two-body processes CC1 + + H~ CH2CI+ + H + HCCI+ + H + HCC12+ CH~ + HCI + CH2C12+
---H2 -H
30.0 -+ 0.5 31.0 +- 0.5 31.5 -+ 0.5 --> 32.0 32.0 -+ 1.0 32.0 - 1.0
6 13 2 0 0 0
3.4 ± 1.0 5.2 -+ 2.0 2.8 +- 1.0 ----
CH~ + CI +
--
32.5 -+ 0.5
61
5.4 -
H H2 H H C1
35.0 35.0 35.5 36.0 37.5
1.0
Three-body processes HCC1 + + H ÷ CCI ÷ + H ÷ CH~ + C1 ÷ CCI ÷ + H~ CH~+H ÷ a
-+ 0.5 -+ 0.5 -+ 1.0 -+ 0.5 -+ 1.0
4 3 9 0 2
3.6 - 2.0 3.6 -+ 2.0 4.3 _+ 1.0 -3.6-+ 2.0
Ref. [11].bRef. [7].
results of refs. [7] and [11] are summarized in Table 1. This paper is divided into the following sections: in Section 2, we describe the methods used to calculate the geometries and energies of the studied molecules; in Section 3 we discuss the double ionization of CH3C1; Section 4 deals with two-body processes while Section 5 is devoted to three-body processes; finally, in Section 6 some conclusions are given.
2. Computational method The computational method used in the present study has already been employed in our previous work [1] on the stability of CH3C12+ and is only briefly summarized. We used the compact effective potentials (CEP) of Stevens and co-workers [12], with the corresponding TZP basis set, i.e. (4spld)/[3spld] for carbon and chlorine. The hydrogen atom was described with the (5slp)/ [3slp] TZP basis set from Dunning [13]. All energy calculations and geometry optimizations for both equilibrium structures and transition states were carried out at the complete active space SCF (CASSCF) level [14] with the HONDO95.3 program package [15]. Harmonic
vibrational frequencies and zero-point energies (ZPEs) were also calculated at the CASSCF level. As already stated in our previous paper [1], the Cerjan and Miller method [16,17] implemented in HONDO95.3 is not efficient for locating transition states and we employed a step by step procedure by freezing the reaction coordinate and optimizing the other ones. This method is sometimes called 'distinguished reaction coordinate'. The effect of dynamical correlation was included via a multi-reference second-order Moller-Plesset perturbative calculation as implemented in the two-class version of the CIPSI program [ 18[ and its diagrammatic version [19,20]. The size of the zeroth-order wave function was about 2000 Slater determinants and the norm of the correction to the wave function was less than 6.4% in all cases. The CASSCF optimized geometries of the various molecules studied in the present work are displayed in Fig. 1 for CH3C1 structures. The geometries of dissociation fragments are not reported here and are available upon request. The corresponding energies are given in Tables 2 and 3. In order to allow comparisons with experimental work (Table 1), all energies are expressed with respect to the neutral chloromethane ground
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
217
Table 2 Calculated energies of the molecular structures involved in the fragmentation of the CH3C12+ dication Na
State
ECASSCF
EciPsI (au)
Norm b
(au)
CH3CI CH3CI ÷ CH3CI 2÷ CH~-C1 ÷ CCI+-H2-H ÷ CH2CI2+-H CH~-H-CI + CH2CIH 2+ CH2CI+-H + HCC12+-H2 HCCI+-H~ CH3C12+ CH 3C12+ CH3CI 2+ CCI+-H~ CH~-HCI +
1 2 38 3b 4a 4b 5 6 7 8a 8b 9 10 11 12 13 14 15 16
0 0 0 0 1 1 1 1 1 0 0 1 1 3 1 1 2 3 1
IAI 2A" ]A' 3A2 tA' 3A2 LA' 3A" IA tA' 3A" ~A' IA' 3A" IA' ~A i ~+ I]2+ IA'
-22.014432 -21.632361 -20.980933 -20.936983 -20.863262 -20.934805 -20.855639 -20.886589 -20.951461 -21.052072 -20.904015 -20.964672 -20.968970 -20.815466 -20.977745 -20.950340 -20.969384 -20.959715 -20.961979
-22.2473343 -21.843796 -21.159529 -21.129397 -21.034041 -21.122848 -21.033560 -21.047200 -21.104938 -21.229535 21.089305 -21.142091 -21.133044 -20.992872 -21.133879 -21.151181 -21.140081 -21.135042 -21.139923
0.027 0.028 0.028 0.045 0.041 0.037 0.019 0.037 0.022 0.027 0.042 0,027 0.024 0.042 0.026 0.023 0.020 0.020 0.029
AEcAsscF
AEcIPS l
~CIPS/ +
(eV)
(eV)
ZPE (eV)
0.00 10.40 28.12 29.32 31.32 29.38 31.53 30.69 28.92 26.19 30.21 28.56 28.44 32.62 28.21 28.95 28.44 28.70 28.64
0.00 10.98 29.60 30.42 33.01 30.60 33.03 32.66 31.09 27.69 31.51 30.07 30.32 34.13 30.30 29.55 30.13 30.27 30.13
0.00 10.74 29.37 29.96 30.40 32.70 32.44 32.25 30.79 27.57 31.17 29.76 29.97 33.65 30.05 29.28 29.75 29.77 29.75
"Number of imaginary frequencies. b Norm of the perturbation correction to the CIPSI wave function.
Table 3 Calculated energies of the fragments appearing in the dissociation of the CH3C12÷ dication
CH 2CI 2+ CH ~-CI ÷ HCCI2+-H CH2CI + HCC12+ H+-CC1 + HCCI + CH~ CH ~ CCI ÷ HC1 ÷ H~ H2 H +2 C1÷ CI
17 18 19 20 21 22 23 24 25 26 27a 27b 28 29 30 31a 31b 32
State
Ecasscv (au)
Ecmsi (au)
Norm a
ZPE (eV)
2B t 2B i 2A' tAi ~+ rE+ 2A' IA' t 2A i ]Yz+ zII 2~+ IA'l I + r,g 2 + ~g 3p ID 2p
-20.386364 -20.303912 -20.312068 -21.079928 -19.811618 -19.702783 -20.408476 -6.925925 -6.238666 -19.812753 - 14.862829 -14.721608 -1.332855
-20.560163 -20.477894 -20.465670 -21.276078 -19.963756 -19.865249 -20.593239 -6.996467 -6.297844 -19.988587 - 14.992339 -14.853978 -1.337310 b -1.167437 b ~).600084 b -14.335546 -14.273332 - 14.789370
0.020 0.016 0.016 0.021 0.016 0.014 0.030 0.002 0.001 0,018 0.003 0.002
0.58 0.51 0.36 0.67 0.35 0.11 0.32 0.84 0.44 0.07 0.16 0.17 0.56 0.27 0.15
-14.215823 -14.149629 - 14.649503
Norm of the perturbation correction to the CIPSI wave function. b Full CI result.
0.001 0.001 0.001
218
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
H ~.103
H,~06.3
~ C
1.827 CI
H
-~ +
1.100 ~ " " ~ _
08.2
1.870
H2
H
103,5 1.108
H3
HCH = 110.7 H2CH3 = 115.1
(2)
(1)
- 7 ++
HI
1.120 ~ ,
1.580 Cl
C '
C
1.110
Cl
CI
Hz ~ - - - , J 1 1 6 . 5 H~3~
1.416
H ~
H34Y
'
H2CH3 = 36.0
HCH = 110.4
(3a)
(3b)
H2CH3 = 57.3 (4a)
÷÷
++
I-I~
2.700
1.212
...1
179.0
HI
4.2oo~io8 o
H,
4 "
o 14
HCH - 116.2 (4b)
12
1.128 H2
~
1.353
127.0
68"1 1.656
+4-
J1181
H 3.O" 1.137
HzCH3 = 14.3
H2CH3 = 123.8
(5)
C6)
+4"
H3
--]
++
1.118
120.1 101.4 H3(120.5) (102. "~CI 1.701
1.118 (1.093)
(1'.313)
Ha 114.3 (1.673) (115.1)
H2CH3 - 2.1 H2CHI = 16.0 (7)
(8a)
Fig. 1. CASSCF optimized geometrics for the molecules involved in the fragmentation of CH 3C12+(when available, the geometries of ref. [2] are given in parentheses).
219
D. Duflot et al./International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
++
H,
jf
98 1
-7
1.333
++
C (=-C: I" H ~ "';-'-"~ 1.728 3..~'~/J121.3 1.738 H2 118.4
1.209
H2CH3- 87.6 H2CH3= 100.5
.-I
120.2 A f (123.8/2,614 (21643) (1.082) )=~ ~C( 1.648 1.110 H2fl"~6.6 (1.626) (1.086) (117.0)
(8b)
(9)
-7 +4.
"~ ++
H~ 160.3 1.119 "~'---~C 1.611CI
HI 165.4 1.132 "~---'~C 1'499CI
H3 d
....
H~
1.583
/
(11)
1.185 HI
101.0
H2 1.399
H2CH3 = 26.7
(10)
N22
136.9 fC ~--,v-- Cl
2"130
1.407 H3
H2CH5 = 20.4
1.226 H3
1.124
Hz ~ - - - J 1 0 5 . 3
H2 ~ ~ 2 . 2 0 0 " ~ 109.9 H3~"
+4=
(12)
-7 ++
~6,2
-7 ÷4.
CI H3 Ha H~ C CI 0.766 1.417 1.185 1.493
1.611 H2
(14)
(13)
HI H2 H3 1.800 0.816 0.873
C
-•++
1.524
CI
105,7 1.119 H5 (i15.2) (I.093) " ~ . _ ~
/_-~
HI -7 ++
107,3/. (102~.4 341 t"-Ci (1.305) 2.934
1.119 Hz'l149 (2.401) (1.094) (106.6) (16)
(15)
Fig. 1. (continued)
220
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
state IAI, whose CIPSI energy was calculated to be -22.247 343 au (Table 2), after zero-point energy (ZPE) correction. The detailed calct~lated harmonic vibrational frequencies for each species are available upon request.
3. Double ionization of CH3C! To our knowledge, the only determination of the double ionization threshold of CH3C1 has been done using the double charge transfer (DCT) method [21]: OH + (3 ~ - ) + CH 3C1 ---* OH- (1 ~ + ) + CH 3C12 ÷
(1) Following the spin conservation rules which seem to hold in DCT spectrometry [22], the use of OH ÷ as collider implies that only triplet states of CH3C12÷ can be observed. In ref. [21], two peaks were observed at 31.5 _ 0.5 eV and 33.5 - 0.5 eV.
The calculated vertical ionization spectrum of CH3C1 is shown in Table 4. The ionization energy is the calculated CIPSI energy difference between the neutral ~A~ state and the dication states at the CASSCF geometry of CH3CI(~A~). The single ionization threshold is calculated to be 11.03 eV, which is close to the recommended value [23] of 11.22 eV. The results for dicationic states were obtained with the CASSCF orbitals of the CH3CI 2+ first singlet state at the equilibrium geometry of the neutral molecule (structure 1 in Fig. 1). For the 3A 2 CH3C12÷ first triplet state, the agreement with experiment is correct: our value of 30.69 eV is slightly below the measured value of 31.5 - 0.5 eV. The agreement for the second peak at 33.5 + 0.5 eV is better, since there are three triplet states lying in the 3 3 - 3 4 eV range, at 33.2, 33.40 and 33.47 eV, respectively (Table 4). In order to test the influence of the starting MOs employed, we also calculated the energy of the first 3A 2 state, using its own CASSCF orbitals. The norm of the perturbative correction was then
Table 4 Calculated vertical spectra of the CH 3C12+ and CH3CI +
CH3CI
CH3CI + CH3CI 2+
State
Dominant configurations
EclPSl (au)
Norm"
AEcJPsl
IAI 2E 3A2 IE
0.9311 a~2a~le43a~2e41 0.9211 a ~2a zi I e 43a ~2e 31 0.921 la~2a~lea3a~2e21 0.8611 a~2a~ lea3a~2e21 0.7711 a~2a~le43a~2e21 0.0911 a22a iZle33a~2e31 0.9011 a~2a21e33a~2e3I 0.871 lalZ2a~le33a~2e31 0.9011 a~2a~le33a22e31 0.851 la~2a~lea3a~2e31 0.801 la~2a21e33a~2e31 0.8611a~2a~ le43a~2e31 0.7111 a~2a ~1e 33a~2e 31 0.0911a~2alZl e23a~Z2e41 0.6511 a22a~le33a~2e31 0.131 la22a~lea3a~2e21 0.091 la~2a~le23a~2e41 0.9211 a' 22a' 23a' 21a"Z4a' Z5a' 22a" II 0.9211 a' 22a' 23a' 21a"24a' 25a' Z2a"I I 0.8911 a' Z2a' 23a' 21a"24a' 25a' Z2a"l I 0.8511 a' 22a' 23a' 21a" 24a' Z5a' 21
-22.247343 -21.842107 -21.119331 -21.084602 -21.065642
0.030 0.028 0.051 0.056 0.059
0.00 11.03 30.69 31.64 32.15
-21.028446 -21.022856 -21.019944 -21.017363 -20.991679 -20.972629 -20.951093
0.062 0.061 0.060 0.062 0.061 0.064 0.058
33.17 33.32 33.40 33.47 34.17 34.69 35.27
-20.918135
0.062
36.17
-21.843786 -21.124619 -21.084516 -21.081736
0.02~ 0.038 0.038 0.033
0.00 19.57 20.66 20.74
ZAI ~A2 3E 3A t 3E 3A 2 IE IE LA~
CH 3C1+ CH 3C12+
2A" 3A,, IA, IA ,
a Norm of the perturbation correction to the CIPSI wave function.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
0.041 instead of 0.051, but the energy remained almost unchanged (30.60 eV). A discrepancy between experimental and theoretical ionization energies is commonly observed for various molecules and methods (see ref. [24] and references cited therein for some examples). Among others, a typical case is C2H~+: while the DCT value [25] of 32.7 ___ 0.3 eV is above the coincidence value at 31.7 eV [24,26], the calculated threshold is 31.35eV using the Green function method [27] and 32.0 eV [26] using the CIPSI program as in the present work. However, the same CIPSI program gave ionization values about 1 eV too low in the case of CO~ +, OCS 2÷ and CS~ + [28], when compared with experimental determinations [29]. The same behavior is observed in the present case for CH3C12+. Clearly, new experiments and calculations to study the double ionization of CH3C12+ are needed to examine more closely this problem. Another remarkable feature in Table 1 is the lack of CH3C12+ signal in both experiments in refs. [7,11]. This could indicate that the double ionization threshold of CH3C12÷ is above the lowest energy barrier for direct dissociation of the dication (in the present case, the CCI+/H~ fragment pair at 30.0 _+ 0.5 eV). However, as discussed below in Section 4.5, it is possible that the CCI+/H~ pair comes from the dissociation of an excited state of CH3C1 ÷ and it is thus not possible to conclude without ambiguity. The CH3C12+ dication may also be obtained by single ionization of the monocation CH3C1 ÷. The experimental determinations are 20.2 ___ 0.5 eV (DCT) [21] and 18.16 _ 0.03 eV (charge stripping) [30]. The result obtained at MP3/6-31G**// UHF/6-31G* level by Yates and co-workers [2] is 21.5 eV. The low value obtained in charge stripping experiment was explained by an excitation to a Rydberg excited state of the monocation with ejection of an electron, followed by an isomerization to the more stable CH2CIH 2+ geometry, as already proposed [30] for CH3F. Thus,
221
the CH3C12+ isomer would not be produced in charge stripping experiments. The three lowest states of the vertical spectrum of CH3C1 ÷ are displayed in Table 4. We used the geometry of the 2A" state (structure 2 in Fig. 1), which was found [1] to be the ground state of CH3C1 + by only 0.15 eV, due to a very weak Jahn-Teller splitting. The 3A" value at 19.57 eV is slightly below the DCT determination [21] at 20.2 ___ 0.5 eV. The ~A' value at 20.74 eV is also below the theoretical result of Yates and co-workers [2] at 21.5 eV. However, these values are well above the charge stripping value of 18.16 + 0.03 eV. Although it is not possible to discuss here the implication of a Rydberg state, it seems clear that the CH3CI 2+ isomer is not produced in this type of experiment.
4. Two-body processes 4.1. CH3CI 2+ ---* CH~ + Cl +
This dissociation pathway is the most important observed in the PEPIPICO experiments of Ruhl and co-workers [7] since the CH~/C1 + pair represents 61% of the fragment products with a kinetic energy release (KER) of 5.4 +_ 1.0 eV (Table 1). In order to study this dissociation channel, the C-C1 distance of the equilibrium structures of the 1A' and 3A2 lowest states of CH3C12+ was progressively increased to obtain the geometry of the corresponding transition states. The results are shown in Fig. 1 (4a and 4b for 1A2 states, respectively) for the geometries and in Table 2 for the energies. Using the calculated ZPE corrections leads to the following activation barriers for the two reactions: CH3C12+ (lA') ---. CH 3 - _ C12+ (lA,)
(2) ---~CH~- (~Al ') + C1 + (lD)
32.70 eV
CH3C12 + (3 A2 ) ----,CH 3 _ _ C12+ (3A2) (3) ---* CHf (IAl ') + C1 + (3P)
30.40 eV
222
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
It should be noticed that the ~A' ground state of CH3CI 2÷ dissociates into the 1D first excited state of chlorine ion while the 3A 2 CH3C12+ excited state correlates to the 3p C1÷ ground state. Another interesting feature is the large difference between the barrier heights of the two reactions with respect to the equilibrium structures: 3.33 eV for Eq. (2) and only 0.44 eV for Eq. (3). However, this is not surprising since the 1A' and 3A 2 states have very different geometries, as already discussed in our previous paper [1] (see also 3a and 3b in Fig. 1)i the CCI bond length in the 1A' state (1.580 A) is much shorter than in the 3A 2 state (1.720,~), because the la" MO has a very pronounced 7r(CC1) character [1]. Thus, one has to break a double bond to produce the CH~/C1 + pair from the singlet state and only a single bond for the triplet state. These results allow us to predict the appearance threshold of the CH~ and C1+ fragments in coincidence experiments as shown in Fig. 2. From this figure, it is clear that the appearance threshold for the IA' is 32.70 eV, while that of the 3A 2 state is governed by the double ionization energy, i.e. 30.69 eV. The first value is in good agreement with the experimental one at 32.5 + 0.5 eV (Table 1), which indicates that the singlet
state fragmentation is observed. Thus, we conclude that the triplet state must follow another dissociation channel. By using the theoretical energy values and ZPE corrections for CH~ and C1÷, it is also possible to calculate the KER of the dissociation reactions. The obtained results are 6.38 eV for Eq. (2) and 5.96 eV for Eq. (3). Both values are in the experimental range (5.4 ___ 1.0 eV) and it is not possible to discriminate between the two processes. After bombarding neutral CH3C1 with 1 MeV atomic ions, Monce and co-workers [5,6] observed the CH~/C1 ÷ fragments, with a KER of 6.6 +- 0.1 eV. This result is close to the value obtained for the ~A' ground state of CH3C12÷ (6.38 eV). 4.2. CH3C12+ ---* CH2C1 + + H +
This dissociation pathway is the second most important one observed in the PEPIPICO experiments of Ruhl and co-workers [7] with 13% of the fragment products and a measured KER of 5.2 ___ 2.0 eV (Table 1). To produce the CH2CI÷/ H ÷ pair, we calculated the transition state geometries obtained by removing one H atom from the equilibrium geometries of the ~A' and 3A 2 states of CH3C12÷. This led to the 5 and 6
34
32.70
32
I A' 31.64-
31.43
3A 2 3 0 . 6 9 ~
3O
/
29.37
~
29.74
28
r~
26 24.73
24 CH3CI++
CH3+ + CI +
CH2+ + C I +
22 Fig. 2. Energy d i a g r a m for CH3C12÷ ~ CH~ + CI ÷ dissociation.
+1t
D. Duflot et aL/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
223
36.71 36
34.2~-'~ HCCI+e+H + H
34.28 34
~t3~ 32.44
32
I A ' 31.64 3A 2 30.69
188/
/
\
30
\
\
29.74 \29.01 CCI+ + H+ + H2
t~
~33.31
CCI+ + H+ + H + H
CH2+ + CI+ + H
28
26
CH3 C I+ + 24
Fig. 3. Energy diagram for CH2C12+production and dissociations.
Eq. (4) corresponds to the direct dissociation of CH3C12+(IA') in the three CCI+/H2/H + fragments (three-body process, discussed in Section 5). The CH2C12+ dication produced in Eq. (5) is not found in the experiments of ref. [7] but is present as a very weak signal in ref. [11] at 32.0 -+- 1.0 eV (Table 1). This value is in very good agreement with the calculated one at 32.25 eV. The predicted KER would then be very low (0.37 eV). It is probable that most of the CH2C12+ dication
structures shown in Fig. 1, respectively. Mulliken population analysis [31] showed that the reactions were in fact (see Fig. 3): CH3C12 + (1A') ---* CC1 ÷ - - H2 - - H + (1A') ---*CCI+(1E+)+H2(I~:g)+H +
3 2 . 4 4 e V (4)
CH3CI 2+ (3A2) ---. CH2C1 - _ H 2÷ (3A2) ---~ CH2C12+ (2Bl) + H(2S)
32.25 eV
(5)
34
32
I A ' 31.643A230.69-
30
30.79 -- ~
~
o~
30.71 29.76
\
H/-H-'~I+ +H+ +H
/,.o,
28
27.57
r~ 26
~_~ 26.09
24
CH3CI+ +
CH2CIH+ +
CH2CI+ + H+
22
Fig. 4. Energy diagram for CH3C12+~ CH2CI+ + H + dissociation.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
224
dissociates spontaneously. Thus, it is clear that the CH2CI+/H + pair cannot be obtained by eqns (4) and (5). Another possibility is to break the H C1 bond of the methylechloronium dication CHzC1H 2+ (structure 8a in Fig. 1) after a preliminary isomerization of CH3C12+ (structure 7 in Fig. 1). The isomerization reaction has been studied in our previous paper [1] for the 1A' state only, since the triplet transition state was found to dissociate along the C-CI coordinate. For this reason, the non-planar 3A" state of CH2C1H 2+ (structure 8b in Fig. 1), which lies 31.17 eV above the energy reference was not considered in the present study. The resulting dissociation pathway as displayed in Fig. 4 is the following: CH3C12+ (1A ') ---* CH~- - - H -
- C I + (IA)
(6) ---* CHzC1H 2+(IA')
30.79 eV
in the PEPICO ones [11] as a very weak signal. The appearance threshold is difficult to determine and is at least 32.0 eV (Table 1). It is important to notice that coincidence experiments do not detect neutral fragments. Hence, the HCCI 2+ ion could correspond to a three-body dissociation with production of two separate H atoms. However, the geometry of the ~A' state of CH3C12+ (structure 3a in Fig. 1) suggests that this state is a weakly bound complex of H 2 with HCC12+. Thus it should be easier to produce the H2 molecule, as revealed by our calculations: the structure of the corresponding transition state (10) is shown in Fig. 1 and the dissociation pathway in Fig. 5, which shows that the height of the energy barrier is only 0.6 eV: CH3C12+ (JA ,) ---. HCC12+ _ - Hz(1A ') HCClZ+(I~2+)+H2(II2g)
29.97 eV
(8)
CH2C1H 2+ (1A') ---*CH2C1 - - H 2+ (1A')
('7) ---* CH2CI+(1AI)+H +
29.76 eV
Eq. (7) has already been studied theoretically by Yates and co-workers [2] at MP3/6-31G**// RHF/6-31G* level. The geometry of their transition state is shown in parentheses in Fig. 1 (structure 9). The agreement between the two calculations is rather good. The isomerization energy of 2.19 eV is close to the previous value of 2.08 eV [2]. The fragments are 1.48 eV below CH2CIH 2+, which is very similar to the result of ref. [2] (1.53 eV). Finally, it is clear from Fig. 4 that the theoretical appearance threshold is given by the vertical ionization energy of the singlet state, i.e. 31.64 eV. This result is slightly higher than the experimental threshold at 31.0 _+ 0.5 eV (Table 1). The calculated KER is 5.55 eV, in good agreement with the experimental value of 5.2 _+ 2.0 eV (Table 1). 4.3. C H 3 C l 2+ ~
H C C l 2+ + H 2
The HCC1 e+ dication is not detected in the PEPIPICO experiments of ref. [7] but is present
The theoretical appearance threshold is governed by the ionization energy of the singlet state, i.e. 31.64 eV, in good agreement with the experimental value at 32.0 eV. 4.4. CH3Cl 2+ ---* H C C l + + H~
The HCCI+/H~ ion pair is the weakest twobody process observed in PEPIPICO experiments [7], with 2% of the final fragments and a KER of 2.8 _+ 1.0 eV (Table 1). From the theoretical point of view, these fragments may be obtained by removing two H atoms from the 3A 2 C3vsymmetric state of CH3C12+ (Fig. 5): CHC12+ (3A2) ~ HCC1 + - - H~- (3A") ---~HCCI+ (2A') + H~- (2~g)
33.65 eV
(9)
The geometry of the corresponding 3A" transition state is shown in Fig. 1 (structure 11). However, the calculation of the harmonic vibrational frequencies gave three imaginary modes, the largest one (i1358 cm -t) corresponding to the movement of the one H atom out of the symmetry plane of the molecule. Curiously, the CHz/CH3 symmetric stretching mode, which is the reaction coordinate, is very low (i139cm-J). The i195cm -I
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
225
33.65 33.65
34
32"43
32
IA'
31.64-H 2 + + CCI + + H
3A 230.69
1 H + + HCCI + + H
30 ¢0
1 28
H 2 + CCI + + H +
28.13
¢d
H2+ + HCCI ÷ 26
24
CH 3 C I+ + 22 Fig. 5. E n e r g y d i a g r a m f o r C H 3 C I 2+ ---* H C C I -
mode is the antisymmetric stretching. This indicates that the true transition has no symmetry at all. In order to find its geometry, we performed test calculations with a configuration space reduced to quadri-excitations. Unfortunately, we were not able to locate this particular point of the triplet potential energy surface. Thus, we can only say that the appearance threshold for this pair is below 33.65 eV and the KER is below 5.52 eV. These values are consistent with the experimental ones. 4.5. C H 3 C I 2+ ---* CCl + + H~
The production of H~ is the most surprising process occurring in the fragmentation of CH3X 2+ compounds [7,10]. Indeed, it requires the breaking of the three CH bonds and the formation of new H - H bonds. In the case of CH3C1 z+, it represents 6% of the final products [7], which is rather important, with a KER of 3.4 + 1.0 eV and a very low appearance threshold [ 11] of 30.0 + 0.5 eV (Table 1). The formation mechanism of H~ is unknown. In the case of methylamine [10], experiments with deuterated species proved that the H~ ion came from the methyl group rather than the amine one. Thissen [10] proposed that the formation of the dication leads to the formation of an
+ H2 p r o d u c t i o n a n d d i s s o c i a t i o n s .
H2 molecule weakly bound to the carbon atom. The dissociation of this structure would give the H2PrlCNH~÷ fragment pair and, owing to the large protonic affinity of H 2 (4 eV), the possible formation of H~ and CNH~. Thus, such a mechanism might also occur in the case of CH3CI 2+, since, as already seen, the ground 1A' state is a weakly bound complex of H 2 and HCC12+. In order to test this hypothesis and explain the presence of H + 3, it is necessary to determine the geometry of a possible transition state between CH3C12+ and CCI+H~. This is a difficult task since this geometry cannot be simply guessed, contrarily to other mechanisms studied in the present work. The usual method consists of examining the shape of the normal modes of the molecule. Because of a too large computational cost, we limited the search to the ~A' state of CH3C12÷, for which we made SCF test calculations before the CASSCF ones. Indeed, the 3A2 state is not stable at SCF level [1]. The CASSCF harmonic frequencies of CH3C12+(1A ') are very similar to the SCF ones (see ref. [1]): the two lowest ones (377 and 800 cm -~ correspond to the rotation of the weakly bound HzH3 group, leading to the (12) Cssymmetric planar geometry shown in Fig. 1.
226
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Obviously, this structure has one imaginary frequency (i326 cm -~) which is the out-of-plane rotation of the H2H3 group. By increasing the H2CC1 angle, we obtained the (13) C2v-S ymmetric planar structure shown in Fig. 1, which also has one imaginary frequency (i1356cm -~) corresponding to the in-plane HzCC1 bending mode. By increasing order, the next frequencies of CH3C12+(1A') are the following: the 867 cm -I mode is the movement of H1 out of the symmetry plane of CH3C12+, the 969 cm -1 mode is the CH3CH 4 symmetric stretch with its anti-symmetric counterpart at 1121 cm-1; the 1060 cm -~ frequency corresponds to the CC1 stretch. The 1409 cm -~ mode seems more interesting, since it corresponds to a simultaneous increase of the three HCC1 angles. However, SCF calculations failed to locate any stable geometry. Finally, the 2953 cm -1 mode is the CH1 stretching and the 3091 cm -1 mode is the H2CH3 bending mode. Thus, it does not seem possible to produce the H~ ion by following the normal modes of the 1A' state of CH3C12+. However, while exploring the potential energy surface of this state, we found that the linear ~E+ structure depicted in Fig. 1 (structure 14) was stable at both SCF and CASSCF levels. Analysis of its vibrational
frequencies shows that this structure is a saddle point with a doubly degenerated imaginary mode (i269 cm -~) corresponding to the rotation of the H2H 3 group. The most surprising result is that it lies at 29.75 eV, i.e. 0.38 eV above the equilibrium geometry. It is then possible to produce the H~/CC1+ pair by increasing the CH1 bond length (structure 15 in Fig. 1): CH3C12+ (1]~+) ~ CC1 + _ - H f (]~ +) (10) --,CCI+(I~+)+H~-(1A1')
Of course, the structure 15 is not, strictly speaking, a transition state, but it appears that it is rather easy to obtain the H~ ion from CH3C12+(1A'). Tentative calculations to find a true transition state by rotating the H2H3 group in 14 failed or led to 13. The whole dissociation pathway is displayed in Fig. 6. From this figure, the appearance threshold is governed by the singlet ionization energy at 31.64 eV, well above the experimental one at 30.0 -+ 0.5 eV. Similarly, the predicted KER is 5.55 eV which is larger than the experimental value of 3.4 _+ 1.0 eV. Because of these large discrepancies, we suggest that the presence of the H~ ion is not due to a direct dissociation of CH3C12+ as given in Eq. (10) but rather to the
34
32
IA' 31.64-
30.74
3A 2 3 0 . 6 9 30
oj
29.77 eV
+ H2+ + H
29.77 ~ 2 9 . 7 5 29.37
28
r~
26 26.09
24 CH3CI++ H2-HCCI++
CCI+ + 113+
22 Fig. 6. Energy diagram for CH3CI2÷--* CC1÷ + H~ dissociation.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
ionization of a Rydberg excited state of the monocation. This would explain the fact that the appearance threshold of the CCI+/H~ pair is lower than the double ionization energy• Such a mechanism was observed for the first time without ambiguity in the case of H 20 2+fragmentation [32]. More recently, the analysis of the KER distributions (KERD) showed a significant contribution of the indirect processes in the case of CO, 02 and NO dications at an excitation energy of 40.8 eV, owing to the autoionization of the oxygen atom [33]. Concerning CH3C12+, another possibility is the implication of the triplet state, which was not considered here, on condition that its ionization threshold is close to the calculated value of 30.69 eV, rather than the experimental one. 4.6. C H 3 C I 2+ ~
[2] in parentheses. It is remarkable that the C-C1 distance is more than 0.5 A greater in our calculation. The isomerization barriers are also different: 2.18 eV instead of 2.48 eV given in ref. [2]. The two fragments are 1.94 eV below CH2C1H 2+ (1.66 eV in ref. [2]). The whole dissociation pathway is shown in Fig. 7, from which we deduce a theoretical appearance threshold of the ion pair of 31.64 eV in good agreement with the experimental value of 32.0 ± 1.0 eV (Table 1). The predicted KER is 6.00 eV. However, this pair is not present in the PEPIPICO spectrum of Ruhl and co-workers [7].
5. Three-body processes Interpretation of PEPIPICO spectra for threebody processes is much more difficult than for two-body processes [34]. In our previous study [26,35] of the fragmentation of CzH~+, we explained that ab initio calculations alone do not allow determination of the precise mechanisms of the fragmentations processes, because it is necessary to add the theoretical energy of the products to the measured KER to obtain an estimation of the appearance threshold. In the case of
CH~ + HCI +
The CH~/HC1 + pair can be obtained through dissociation of the CHzC1H 2+ after preliminary isomerization of CH3C12+ via Eq. (6): CHzC1H2+ (IA ') ----*CH2 - - C1H2 + (IA ')
(11) --*CHf(2AI)+HCI+(2H)
29.75 eV
The geometry of the transition state (16) is shown in Fig. 1, with the values of Yates and co-workers 34
•
32 ~" 3O
IA' 31.64-
~
\
/
CI+( I
~
~
31"43D)+H(2S)
~
[
\
I 29.41/
~~ ~
It-
[[ 29.74
28 27.57 26
25.63 (2H)
24 CH3CI++
227
CH2CIH++
CH2 + + HCI + CH2+ + CI + + H
22
Fig. 7. Energy diagram for CH3C12+~ CH~ + HCI÷ dissociation.
228
D. Duflot et aL/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
CH3C12+, the situation is even more complex, because of the large number of possible dissociation channels and the low precision of the KER measurements (Table 1). 5.1. C H 3 C I 2+ ---* C H ~ + C I + + H
This dissociation pathway is the most important three-body process observed in ref. [7], since the CH~/C1 + ion pair represents 9% of the observed products with a KER of 4.3 _+ 1.0 eV (Table 1). From the coincidence peak slope, it was deduced that the H atom was formed by decay of CH~. From the theoretical point of view, these fragments can be obtained in different ways: C H f (IA1 ') +CI+(1D) ---. CH~- (1A1 ' ) (12) +CI+(1D)+H(2S)
CH~-(1A 1') +C1 + (3p) ----~C H f (IA'l)
(13)
+ C1 + (3p) + H(2S)
5.2. CH3C12+ ---* H C C I + + H + H +
The HCCl+/H + ion pair represents 4% of the final products in ref. [7] with a KER of 3.6 2.0 eV (Table 1). The slope of the experimental peak does not allow identification of the mechanism of the fragmentations. There are only two possibilities (Figs. 4 and 5): CHzCI+(1A1) + H + ---, HCCI+ (2A') +H(2S) +H+ (16) HCC1 + (2A') + H~- (2~g) ---, HCC1 + (2A')
(17) + H(2S) + H + The appearance threshold is predicted to be 34.3 _+ 2.0eV, in good agreement with the measured one at 35.0 -+ 0.5 eV (Table 1). However, it is not possible to decide which of the two processes occurs in the fragmentation. Quite unexpectedly, the CH 2C12+(2 B 1) dication dissociates in the following way (Fig. 3): CH2 C12+ (2B l) + H( 2S) ---*HCC12 ÷ (1 ~ + )
CH~- (2A1) + HC1 + (21-I) ---* CH~- (2A1) (14) +CI+ (1D) +H(2S)
(18) +H(2S)+H(2S)+H(2S)
34.03eV
CH2C12 + (2B 1) + H(2S) ~ CH~- - - C1 + (2B l ) 5.3. C H 3 C I 2+ ---* H + + C C I + + H 2
+ H(2S) --+ CH2(2A1) + C1 + (ID) + H(2S)
(15)
Eqs. (12) and (13) are depicted in Fig. 2. We can predict an appearance threshold of about 34.0 _+ 1.0 eV for the triplet state and 35.7 _+ 1.0 eV for the singlet one. Both values are consistent with the experimental one at 35-36 eV (Table 1). Eq. (14) is shown in Fig. 7 for the singlet state, with a theoretical threshold of 35.7 _+ 1.0 eV. Finally, Eq. (15), which is a 'deferred charge separation' [32], presents two energy barriers (Fig. 6), the higher one at 34.03 eV, leading to a predicted threshold of 38.3 -+ 1.0 eV, which is obviously too high. The calculations cannot discriminate between the eqns (12)-(14) processes, but the last one Eq. (15) is excluded by experiments [7].
The CCl+/H + ion pair represents 3% of the fragments observed in ref. [7] with a KER of 3.6 _+ 2.0eV (Table 1). Its origin remained ambiguous. Assuming that the two H atoms form a molecule, the first possible dissociation pathway is the 'instantaneous explosion' [32] already shown in Eq. (4) of Section 4.2 (Fig. 3). However, the predicted appearance threshold is 32.44eV, which is below the measured one at 35.5 -+ 0.5 eV (Table 1). Other possible channels are the following (Figs. 4 and 5, respectively): CH2CI+ (IAl) + H + ---, CCI+ (lp. +) (19) +H + +H2(l~;)
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
HCC12+ (1~ + ) + H2(l~g) ---. CCI+ (1 ~ + ) (20) + H + +Hz(lZg) For both processes, the predicted appearance threshold is 32.6 + 2.0 eV, which is below the experimental one at 35.0 _+ 0.5 eV (Table 1). It is also possible to produce two separate H atoms, after production of HCC12+ via Eq. (18), in the following manner (Fig. 3): HCCI2+ (lZ+) + H(2S) +H(2S) ---. H + +CCI+(I~+)+H(zS)+H(2S)
229
3.6 --_ 2.0 eV (Table 1). The coincidence peak slope indicates without ambiguity that the C1 atom is formed by decay of HC1 + into H + + CI. Since the 21-Iground state of HC1 ÷, which correlates to the ~A' and 3A" lowest states of CH2C1H 2+, dissociates into H + C1+ as shown in Eq. (14), this ion pair must come from an excited state of HC1 ÷. A possible candidate is the first one, i.e the 2~:+ state. The dissociation pathway is the following, after isomerization to produce CH2C1H 2+ (Fig. 7): C H f (2AI) + HC1 + (2]S+ ) -.--,CH~- (2A1)
36.71eV
(24) (21)
Thus, we have the very remarkable series of deferred charge separations for the 3A2 state of CH3C12+: CH3C12+ ~ C H z C I 2+ --, HCCI 2+ ~ H + + CC1 +. For Eq. (21), the predicted appearance threshold is 36.71 eV, which is above the experimental one, although the calculated KER of 3.4 eV is close to the measured one.
+ H + + CI(2p) In this case, the final products have an energy of 30.99 eV, and the predicted appearance threshold is about 34.6 _+ 2.0eV, which is below the experimental one at 37.5 + 1.0eV (Table 1). Because of the large uncertainties, it is not possible to give more precise information on the origin of this ion pair.
5.4. CH3CI e+ ---, H + CCI ÷ + H~
The CCI+/H~ ion pair is not detected in the PEPIPICO experiments of ref. [7] but is present in the PEPICO ones [11] with an appearance threshold of 36.0 _ 0.5 eV (Table 1). The possible dissociation pathways are the following (Figs. 5 and 6): HCC1 + (2A') + H~- (2~g) --, H(2S) (22) +CCl+(l~+) +H2(2~g ) CCl+ (J £ + ) + H~-(IAl ') ---, H(2S) (23) The calculated energy of the final fragments (30.74 eV) combined with the measured appearance threshold gives a predicted KER of 5.26 _+ 0.5 eV. 5.5. CH3CI 2+ ---, CH~ + H + + CI
The CH~/H + ion pair represents 2% of the fragments observed in ref. [7] with a KER of
6. Conclusion In this paper, we have used large scale ab initio calculations to study the fragmentation processes of the chloromethane dication. This study was made difficult for several reasons. Except the CH~/CI+ and CH2CI+/H ÷ dissociation pathways representing 74% of the total intensity, all other channels contribute individually to less than 10%, especially the three-body processes. All two-body dissociations occur in a very small energy range centered around 31 eV and the uncertainties of the available experiments do not allow a good discrimination between the processes. Nevertheless, our calculations have explained most of the two-body thresholds by a direct dissociation of the lowest singlet and triplet states of CH3C12+, taking into account the experimental uncertainties. It appears that the production of
230
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
the H~ ion is due to an indirect process, involving the ionization of a Rydberg excited state of the monocation. For three-body processes, the large uncertainty on the measured KER and the number of possible channels does not allow us to determine precisely the fragmentation pathway in most cases. More precise experiments, combined with dynamical calculations (using RRKM theory for example) are needed to understand in detail the fragmentation of CH3C12+. Another problem is the calculation of the double ionization threshold of molecules. From the computational point of view, the present work shows that the study of the CH3-X 2+ dication family represents a challenge for ab initio calculations. The marginal stability of the species requires the use of extended basis sets combined with correlated methods, as already shown [1]. The use of the CASSCF method is well suited for studying dissociation processes. However, as illustrated in the present paper, the computational cost does not allow us to explore fully the potential energy surfaces of the molecule. It does not seem possible to apply this method to larger C H 3 - X 2+ dications, such as methylamine or methanol [7]. The problem is then to find a good compromise between accuracy and computational time.
Acknowledgements We are very grateful to Dr Roland Thissen for providing us with his unpublished experimental results and for reading carefully this manuscript. The Laboratoire de Dynamique MolEculaire et Photonique is 'Unite de Recherche AssociEe au CNRS'. The Centre d'Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Fonds EuropEen de D6veloppement Economique des REgions. The authors acknowledge the CNRS (Mathematical and Physical Science Department) and the MinistEre charge de la Recherche for a generous allocation of computer time on the IBM RISC 6000 at IDRIS (Orsay).
References [1] D. Duflot, J-M. Robbe, J-P. Flament, J. Chem. Phys. 103 (1995) 19571. [2] B.F. Yates, W.J. Bouma, L. Radom, J. Am. Chem. Soc 108 (1986) 6545. [3] H. Jahn, E. Teller, Proc. R. Soc. London, Ser. A 161 (1937) 220. [4] M.J. Riley, A. Furlan, Chem. Phys. 210 (1996) 389. [5] M.N. Monce, A.K. Edwards, R.M. Wood, F.M. Steuer, A.V. Shah, P. Tabor, J. Chem. Phys. 74 (1981) 2860. [6] M.N. Monce, A.K. Edwards, R.M. Wood, F.M. Steuer, A.V. Shah, J. Chem. Phys. 78 (1983) 597. [7] E. Ruhl, S.D. Price, S. Leach, J.H.D. Eland, Int. J. Mass Spectrom. Ion P,'ocess. 97 (1990) 175. [8] F.S. Wort, R.N. Royds, J.H.D. Elan& J. Electron Spectrosc. Relat. Phenom. 41 (1986) 297. [9] R. Thissen, Ph.D. Thesis, University of Liege, Belgium, 1993. [10] R. Thissen, M. Simon, M.-J. Hubin-Franskin, J. Chem. Phys. 101 (1994) 7548. [11] R. Thissen, unpublished results. [12] W.J. Stevens, H. Basch, M. Krauss, J. Chem. Phys. 81 (1984) 6026. [13] T.H. Dunning, J. Chem. Phys., 55 (1971) 716, 3958. [14] B.O. Roos, Adv. Chem. Phys. 69 (1987) 399. [15] M. Dupuis, A. Marquez, E.R. Davidson, HONDO95.3 from CHEM-Station, IBM Corporation, Neighborhood Road, Kingston, NY 12401, 1995. [16] C.J. Cerjan, W.H. Miller, J. Chem. Phys. 75 (1981) 2800. [17] J. Simons, P. J0rgensen, H. Taylor, J. Ozment, J. Chem. Phys. 87 (1983) 2745. [18] B. Huron, J-P. Malrieu, P. Rancurel, J. Chem. Phys. 58 (1973) 5745. [19] R. Cimiraglia, J. Chem. Phys. 83 (1985) 1746. [20] R. Cimiraglia, M. Persico, J. Comput. Chem. 8 (1987) 39. [21] W.J. Griffiths, F.M. Harris, Rapid Commun. Mass Spectrom. 2 (1988) 91. [22] F.M. Harris, Int. J. Mass Spectrom. Ion Process. 120 (1992) 1. [23] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin, W.G. Mallard, J. Phys. Chem. Ref. Data, Suppl., 17 (1988). [24] R.I. Hall, L. Avaldi, G. Dawber, A.G. McConkey, M.A. McDonald, G.C. King, Chem. Phys. 187 (1994) 125. [25] S.R. An~ews, FNI. Hairis, D.E. Parry, Chem. Phys. 166 (1992) 69. [26] E.M.-L. Ohrendorf, F. Tarantelli, L.S. Cederbaum, J. Chem. Phys. 92 (1990) 2984. [27] R. Thissen, J. Delwiche, J-M. Robbe, D. Duflot, J-P. Flament, J.H.D. Eland, J. Chem. Phys. 99 (1993) 6590. [28] P. Millie, I. Nenner, P. Archirel, P. Lablanquie, P. Fournier, J.H.D. Eland, J. Chem. Phys. 84 (1986) 1259. [29] M.L. Langford, F.M. Harris, C.J. Reid, J.A. Ballantine, D.E. Parry, Chem. Phys. 149 (1991) 445. [30] F. Maquin, D. Stahl, A. Sawaryn, P.v.R. Schleyer, W. Koch, G. Frenking, H. Schwartz,J. Chem. Soc., ~ Commun., 504 (1985). [31] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833. [32] D. Winkoun, G. Dujardin, L. Hellner, M.J. Besnard, J. Phys. B 21 (1985) 1385. [33] S. Hsieh, J.H.D. Eland, J. Phys. B 29 (1996) 5795. [34] J.H.D. Eland, Mol. Phys. 61 (1987) 725. [35] D. Duflot, J-M. Robbe, J-P. Flament, J. Chem. Phys. 102 (1995) 355.
ELS EVI E R
International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Ab initio study of the chloromethane dication fragmentation Denis Duflot*, Jean-Michel Robbe, Jean-Pierre Flament Laboratoire de Dynamique Moldculaire et Photonique, URA CNRS 779. Centre d'Etudes et de Recherches Lasers et Applications, Universit( des Sciences et Technologies de Lille, UFR de Physique, 59655 Villeneuve d'Ascq Cedex, France
Received 7 May 1997; revised 20 June 1997; accepted 6 July 1997
Abstract The chloromethane dication dissociation pathways have been studied using the complete active space self-consistent field (CASSCF) method followed by a multi-reference perturbative configuration interaction (CI). The vertical double ionization energy is calculated to be 31.64 eV for the first singlet state and 30.69 eV for the first triplet state, the latter value being close to the experimental determination of 31.5 -+ 0.5 eV. For most two-body dissociation processes, the appearance thresholds of the ion pairs observed in coincidence experiments are in good agreement with the results of the calculations. In the case of the CCI+/H~ pair, the dissociation is indirect and comes from the ionization of a Rydberg excited state of the CH3CI ÷ monocation. For three-body processes, the uncertainties of the available experimental results and the large number of possible dissociation channels do not allow identification uniquely of the involved mechanisms in most cases. © 1997 Elsevier Science B.V. K e y w o r d s : Configuration interaction; Chloromethane; Dications; Fragmentation
1. Introduction
In a previous paper [1], we used large scale ab initio techniques to study the stability of the two lowest states of the CH3C12+ dication. We found that the 3A 2 and IA' states were stable, contrary to the calculations performed at Hartree-Fock level with a 6-31G* basis set by Yates and coworkers [2]. While t h e 3A 2 state keeps the C3v character of the neutral species, the ~A' state is distorted because of an important Jahn-Teller effect [3,4]. In the present work, we present a theoretical interpretation of the available experimental data on the fragmentation of CH3C12÷. The CH3C12+ fragmentation has been the * Tel: 00 33 3 20 43 66 96; Fax: 00 33 3 20 43 40 85; E-mail: duflot @lsm350.univ-lille I .fr
subject of only a few experimental studies. Some years ago, Monce and co-workers [5,6] observed the CH~ and C1+ fragments after bombarding neutral CH3CI with 1 MeV H +, He + and O + projectiles. The authors deduced from coincidence measurements that these fragments came from the dissociation of a CH3C12+ precursor. More recently, Ruhl and co-workers [7] used the PEPIPICO method (photoelectron-photoionphotoion coincidence) [8] to obtain the relative abundance and the kinetic energy release (KER) of each ion pair at an excitation energy of 40.8 eV. The PEPICO method (photoeelectronphotoion coincidence) combined with synchrotron radiation has also been used to study the excitation of the Cl(2p) shell [9,10] and to obtain the appearance thresholds of the fragmentation products at low energies [11]. The main
0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 1 1 7 6 ( 9 7 ) 0 0 1 2 3 - 7
216
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Table 1 S u m m a r y of experimental results on the fragmentation of the CH3C12+ dication Products
Neutral
Threshold a
Intensity b
KER b
(eV)
(% pairs at 40.8 eV)
(40.8 eV)
Two-body processes CC1 + + H~ CH2CI+ + H + HCCI+ + H + HCC12+ CH~ + HCI + CH2C12+
---H2 -H
30.0 -+ 0.5 31.0 +- 0.5 31.5 -+ 0.5 --> 32.0 32.0 -+ 1.0 32.0 - 1.0
6 13 2 0 0 0
3.4 ± 1.0 5.2 -+ 2.0 2.8 +- 1.0 ----
CH~ + CI +
--
32.5 -+ 0.5
61
5.4 -
H H2 H H C1
35.0 35.0 35.5 36.0 37.5
1.0
Three-body processes HCC1 + + H ÷ CCI ÷ + H ÷ CH~ + C1 ÷ CCI ÷ + H~ CH~+H ÷ a
-+ 0.5 -+ 0.5 -+ 1.0 -+ 0.5 -+ 1.0
4 3 9 0 2
3.6 - 2.0 3.6 -+ 2.0 4.3 _+ 1.0 -3.6-+ 2.0
Ref. [11].bRef. [7].
results of refs. [7] and [11] are summarized in Table 1. This paper is divided into the following sections: in Section 2, we describe the methods used to calculate the geometries and energies of the studied molecules; in Section 3 we discuss the double ionization of CH3C1; Section 4 deals with two-body processes while Section 5 is devoted to three-body processes; finally, in Section 6 some conclusions are given.
2. Computational method The computational method used in the present study has already been employed in our previous work [1] on the stability of CH3C12+ and is only briefly summarized. We used the compact effective potentials (CEP) of Stevens and co-workers [12], with the corresponding TZP basis set, i.e. (4spld)/[3spld] for carbon and chlorine. The hydrogen atom was described with the (5slp)/ [3slp] TZP basis set from Dunning [13]. All energy calculations and geometry optimizations for both equilibrium structures and transition states were carried out at the complete active space SCF (CASSCF) level [14] with the HONDO95.3 program package [15]. Harmonic
vibrational frequencies and zero-point energies (ZPEs) were also calculated at the CASSCF level. As already stated in our previous paper [1], the Cerjan and Miller method [16,17] implemented in HONDO95.3 is not efficient for locating transition states and we employed a step by step procedure by freezing the reaction coordinate and optimizing the other ones. This method is sometimes called 'distinguished reaction coordinate'. The effect of dynamical correlation was included via a multi-reference second-order Moller-Plesset perturbative calculation as implemented in the two-class version of the CIPSI program [ 18[ and its diagrammatic version [19,20]. The size of the zeroth-order wave function was about 2000 Slater determinants and the norm of the correction to the wave function was less than 6.4% in all cases. The CASSCF optimized geometries of the various molecules studied in the present work are displayed in Fig. 1 for CH3C1 structures. The geometries of dissociation fragments are not reported here and are available upon request. The corresponding energies are given in Tables 2 and 3. In order to allow comparisons with experimental work (Table 1), all energies are expressed with respect to the neutral chloromethane ground
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
217
Table 2 Calculated energies of the molecular structures involved in the fragmentation of the CH3C12+ dication Na
State
ECASSCF
EciPsI (au)
Norm b
(au)
CH3CI CH3CI ÷ CH3CI 2÷ CH~-C1 ÷ CCI+-H2-H ÷ CH2CI2+-H CH~-H-CI + CH2CIH 2+ CH2CI+-H + HCC12+-H2 HCCI+-H~ CH3C12+ CH 3C12+ CH3CI 2+ CCI+-H~ CH~-HCI +
1 2 38 3b 4a 4b 5 6 7 8a 8b 9 10 11 12 13 14 15 16
0 0 0 0 1 1 1 1 1 0 0 1 1 3 1 1 2 3 1
IAI 2A" ]A' 3A2 tA' 3A2 LA' 3A" IA tA' 3A" ~A' IA' 3A" IA' ~A i ~+ I]2+ IA'
-22.014432 -21.632361 -20.980933 -20.936983 -20.863262 -20.934805 -20.855639 -20.886589 -20.951461 -21.052072 -20.904015 -20.964672 -20.968970 -20.815466 -20.977745 -20.950340 -20.969384 -20.959715 -20.961979
-22.2473343 -21.843796 -21.159529 -21.129397 -21.034041 -21.122848 -21.033560 -21.047200 -21.104938 -21.229535 21.089305 -21.142091 -21.133044 -20.992872 -21.133879 -21.151181 -21.140081 -21.135042 -21.139923
0.027 0.028 0.028 0.045 0.041 0.037 0.019 0.037 0.022 0.027 0.042 0,027 0.024 0.042 0.026 0.023 0.020 0.020 0.029
AEcAsscF
AEcIPS l
~CIPS/ +
(eV)
(eV)
ZPE (eV)
0.00 10.40 28.12 29.32 31.32 29.38 31.53 30.69 28.92 26.19 30.21 28.56 28.44 32.62 28.21 28.95 28.44 28.70 28.64
0.00 10.98 29.60 30.42 33.01 30.60 33.03 32.66 31.09 27.69 31.51 30.07 30.32 34.13 30.30 29.55 30.13 30.27 30.13
0.00 10.74 29.37 29.96 30.40 32.70 32.44 32.25 30.79 27.57 31.17 29.76 29.97 33.65 30.05 29.28 29.75 29.77 29.75
"Number of imaginary frequencies. b Norm of the perturbation correction to the CIPSI wave function.
Table 3 Calculated energies of the fragments appearing in the dissociation of the CH3C12÷ dication
CH 2CI 2+ CH ~-CI ÷ HCCI2+-H CH2CI + HCC12+ H+-CC1 + HCCI + CH~ CH ~ CCI ÷ HC1 ÷ H~ H2 H +2 C1÷ CI
17 18 19 20 21 22 23 24 25 26 27a 27b 28 29 30 31a 31b 32
State
Ecasscv (au)
Ecmsi (au)
Norm a
ZPE (eV)
2B t 2B i 2A' tAi ~+ rE+ 2A' IA' t 2A i ]Yz+ zII 2~+ IA'l I + r,g 2 + ~g 3p ID 2p
-20.386364 -20.303912 -20.312068 -21.079928 -19.811618 -19.702783 -20.408476 -6.925925 -6.238666 -19.812753 - 14.862829 -14.721608 -1.332855
-20.560163 -20.477894 -20.465670 -21.276078 -19.963756 -19.865249 -20.593239 -6.996467 -6.297844 -19.988587 - 14.992339 -14.853978 -1.337310 b -1.167437 b ~).600084 b -14.335546 -14.273332 - 14.789370
0.020 0.016 0.016 0.021 0.016 0.014 0.030 0.002 0.001 0,018 0.003 0.002
0.58 0.51 0.36 0.67 0.35 0.11 0.32 0.84 0.44 0.07 0.16 0.17 0.56 0.27 0.15
-14.215823 -14.149629 - 14.649503
Norm of the perturbation correction to the CIPSI wave function. b Full CI result.
0.001 0.001 0.001
218
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
H ~.103
H,~06.3
~ C
1.827 CI
H
-~ +
1.100 ~ " " ~ _
08.2
1.870
H2
H
103,5 1.108
H3
HCH = 110.7 H2CH3 = 115.1
(2)
(1)
- 7 ++
HI
1.120 ~ ,
1.580 Cl
C '
C
1.110
Cl
CI
Hz ~ - - - , J 1 1 6 . 5 H~3~
1.416
H ~
H34Y
'
H2CH3 = 36.0
HCH = 110.4
(3a)
(3b)
H2CH3 = 57.3 (4a)
÷÷
++
I-I~
2.700
1.212
...1
179.0
HI
4.2oo~io8 o
H,
4 "
o 14
HCH - 116.2 (4b)
12
1.128 H2
~
1.353
127.0
68"1 1.656
+4-
J1181
H 3.O" 1.137
HzCH3 = 14.3
H2CH3 = 123.8
(5)
C6)
+4"
H3
--]
++
1.118
120.1 101.4 H3(120.5) (102. "~CI 1.701
1.118 (1.093)
(1'.313)
Ha 114.3 (1.673) (115.1)
H2CH3 - 2.1 H2CHI = 16.0 (7)
(8a)
Fig. 1. CASSCF optimized geometrics for the molecules involved in the fragmentation of CH 3C12+(when available, the geometries of ref. [2] are given in parentheses).
219
D. Duflot et al./International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
++
H,
jf
98 1
-7
1.333
++
C (=-C: I" H ~ "';-'-"~ 1.728 3..~'~/J121.3 1.738 H2 118.4
1.209
H2CH3- 87.6 H2CH3= 100.5
.-I
120.2 A f (123.8/2,614 (21643) (1.082) )=~ ~C( 1.648 1.110 H2fl"~6.6 (1.626) (1.086) (117.0)
(8b)
(9)
-7 +4.
"~ ++
H~ 160.3 1.119 "~'---~C 1.611CI
HI 165.4 1.132 "~---'~C 1'499CI
H3 d
....
H~
1.583
/
(11)
1.185 HI
101.0
H2 1.399
H2CH3 = 26.7
(10)
N22
136.9 fC ~--,v-- Cl
2"130
1.407 H3
H2CH5 = 20.4
1.226 H3
1.124
Hz ~ - - - J 1 0 5 . 3
H2 ~ ~ 2 . 2 0 0 " ~ 109.9 H3~"
+4=
(12)
-7 ++
~6,2
-7 ÷4.
CI H3 Ha H~ C CI 0.766 1.417 1.185 1.493
1.611 H2
(14)
(13)
HI H2 H3 1.800 0.816 0.873
C
-•++
1.524
CI
105,7 1.119 H5 (i15.2) (I.093) " ~ . _ ~
/_-~
HI -7 ++
107,3/. (102~.4 341 t"-Ci (1.305) 2.934
1.119 Hz'l149 (2.401) (1.094) (106.6) (16)
(15)
Fig. 1. (continued)
220
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
state IAI, whose CIPSI energy was calculated to be -22.247 343 au (Table 2), after zero-point energy (ZPE) correction. The detailed calct~lated harmonic vibrational frequencies for each species are available upon request.
3. Double ionization of CH3C! To our knowledge, the only determination of the double ionization threshold of CH3C1 has been done using the double charge transfer (DCT) method [21]: OH + (3 ~ - ) + CH 3C1 ---* OH- (1 ~ + ) + CH 3C12 ÷
(1) Following the spin conservation rules which seem to hold in DCT spectrometry [22], the use of OH ÷ as collider implies that only triplet states of CH3C12÷ can be observed. In ref. [21], two peaks were observed at 31.5 _ 0.5 eV and 33.5 - 0.5 eV.
The calculated vertical ionization spectrum of CH3C1 is shown in Table 4. The ionization energy is the calculated CIPSI energy difference between the neutral ~A~ state and the dication states at the CASSCF geometry of CH3CI(~A~). The single ionization threshold is calculated to be 11.03 eV, which is close to the recommended value [23] of 11.22 eV. The results for dicationic states were obtained with the CASSCF orbitals of the CH3CI 2+ first singlet state at the equilibrium geometry of the neutral molecule (structure 1 in Fig. 1). For the 3A 2 CH3C12÷ first triplet state, the agreement with experiment is correct: our value of 30.69 eV is slightly below the measured value of 31.5 - 0.5 eV. The agreement for the second peak at 33.5 + 0.5 eV is better, since there are three triplet states lying in the 3 3 - 3 4 eV range, at 33.2, 33.40 and 33.47 eV, respectively (Table 4). In order to test the influence of the starting MOs employed, we also calculated the energy of the first 3A 2 state, using its own CASSCF orbitals. The norm of the perturbative correction was then
Table 4 Calculated vertical spectra of the CH 3C12+ and CH3CI +
CH3CI
CH3CI + CH3CI 2+
State
Dominant configurations
EclPSl (au)
Norm"
AEcJPsl
IAI 2E 3A2 IE
0.9311 a~2a~le43a~2e41 0.9211 a ~2a zi I e 43a ~2e 31 0.921 la~2a~lea3a~2e21 0.8611 a~2a~ lea3a~2e21 0.7711 a~2a~le43a~2e21 0.0911 a22a iZle33a~2e31 0.9011 a~2a21e33a~2e3I 0.871 lalZ2a~le33a~2e31 0.9011 a~2a~le33a22e31 0.851 la~2a~lea3a~2e31 0.801 la~2a21e33a~2e31 0.8611a~2a~ le43a~2e31 0.7111 a~2a ~1e 33a~2e 31 0.0911a~2alZl e23a~Z2e41 0.6511 a22a~le33a~2e31 0.131 la22a~lea3a~2e21 0.091 la~2a~le23a~2e41 0.9211 a' 22a' 23a' 21a"Z4a' Z5a' 22a" II 0.9211 a' 22a' 23a' 21a"24a' 25a' Z2a"I I 0.8911 a' Z2a' 23a' 21a"24a' 25a' Z2a"l I 0.8511 a' 22a' 23a' 21a" 24a' Z5a' 21
-22.247343 -21.842107 -21.119331 -21.084602 -21.065642
0.030 0.028 0.051 0.056 0.059
0.00 11.03 30.69 31.64 32.15
-21.028446 -21.022856 -21.019944 -21.017363 -20.991679 -20.972629 -20.951093
0.062 0.061 0.060 0.062 0.061 0.064 0.058
33.17 33.32 33.40 33.47 34.17 34.69 35.27
-20.918135
0.062
36.17
-21.843786 -21.124619 -21.084516 -21.081736
0.02~ 0.038 0.038 0.033
0.00 19.57 20.66 20.74
ZAI ~A2 3E 3A t 3E 3A 2 IE IE LA~
CH 3C1+ CH 3C12+
2A" 3A,, IA, IA ,
a Norm of the perturbation correction to the CIPSI wave function.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
0.041 instead of 0.051, but the energy remained almost unchanged (30.60 eV). A discrepancy between experimental and theoretical ionization energies is commonly observed for various molecules and methods (see ref. [24] and references cited therein for some examples). Among others, a typical case is C2H~+: while the DCT value [25] of 32.7 ___ 0.3 eV is above the coincidence value at 31.7 eV [24,26], the calculated threshold is 31.35eV using the Green function method [27] and 32.0 eV [26] using the CIPSI program as in the present work. However, the same CIPSI program gave ionization values about 1 eV too low in the case of CO~ +, OCS 2÷ and CS~ + [28], when compared with experimental determinations [29]. The same behavior is observed in the present case for CH3C12+. Clearly, new experiments and calculations to study the double ionization of CH3C12+ are needed to examine more closely this problem. Another remarkable feature in Table 1 is the lack of CH3C12+ signal in both experiments in refs. [7,11]. This could indicate that the double ionization threshold of CH3C12÷ is above the lowest energy barrier for direct dissociation of the dication (in the present case, the CCI+/H~ fragment pair at 30.0 _+ 0.5 eV). However, as discussed below in Section 4.5, it is possible that the CCI+/H~ pair comes from the dissociation of an excited state of CH3C1 ÷ and it is thus not possible to conclude without ambiguity. The CH3C12+ dication may also be obtained by single ionization of the monocation CH3C1 ÷. The experimental determinations are 20.2 ___ 0.5 eV (DCT) [21] and 18.16 _ 0.03 eV (charge stripping) [30]. The result obtained at MP3/6-31G**// UHF/6-31G* level by Yates and co-workers [2] is 21.5 eV. The low value obtained in charge stripping experiment was explained by an excitation to a Rydberg excited state of the monocation with ejection of an electron, followed by an isomerization to the more stable CH2CIH 2+ geometry, as already proposed [30] for CH3F. Thus,
221
the CH3C12+ isomer would not be produced in charge stripping experiments. The three lowest states of the vertical spectrum of CH3C1 ÷ are displayed in Table 4. We used the geometry of the 2A" state (structure 2 in Fig. 1), which was found [1] to be the ground state of CH3C1 + by only 0.15 eV, due to a very weak Jahn-Teller splitting. The 3A" value at 19.57 eV is slightly below the DCT determination [21] at 20.2 ___ 0.5 eV. The ~A' value at 20.74 eV is also below the theoretical result of Yates and co-workers [2] at 21.5 eV. However, these values are well above the charge stripping value of 18.16 + 0.03 eV. Although it is not possible to discuss here the implication of a Rydberg state, it seems clear that the CH3CI 2+ isomer is not produced in this type of experiment.
4. Two-body processes 4.1. CH3CI 2+ ---* CH~ + Cl +
This dissociation pathway is the most important observed in the PEPIPICO experiments of Ruhl and co-workers [7] since the CH~/C1 + pair represents 61% of the fragment products with a kinetic energy release (KER) of 5.4 +_ 1.0 eV (Table 1). In order to study this dissociation channel, the C-C1 distance of the equilibrium structures of the 1A' and 3A2 lowest states of CH3C12+ was progressively increased to obtain the geometry of the corresponding transition states. The results are shown in Fig. 1 (4a and 4b for 1A2 states, respectively) for the geometries and in Table 2 for the energies. Using the calculated ZPE corrections leads to the following activation barriers for the two reactions: CH3C12+ (lA') ---. CH 3 - _ C12+ (lA,)
(2) ---~CH~- (~Al ') + C1 + (lD)
32.70 eV
CH3C12 + (3 A2 ) ----,CH 3 _ _ C12+ (3A2) (3) ---* CHf (IAl ') + C1 + (3P)
30.40 eV
222
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
It should be noticed that the ~A' ground state of CH3CI 2÷ dissociates into the 1D first excited state of chlorine ion while the 3A 2 CH3C12+ excited state correlates to the 3p C1÷ ground state. Another interesting feature is the large difference between the barrier heights of the two reactions with respect to the equilibrium structures: 3.33 eV for Eq. (2) and only 0.44 eV for Eq. (3). However, this is not surprising since the 1A' and 3A 2 states have very different geometries, as already discussed in our previous paper [1] (see also 3a and 3b in Fig. 1)i the CCI bond length in the 1A' state (1.580 A) is much shorter than in the 3A 2 state (1.720,~), because the la" MO has a very pronounced 7r(CC1) character [1]. Thus, one has to break a double bond to produce the CH~/C1 + pair from the singlet state and only a single bond for the triplet state. These results allow us to predict the appearance threshold of the CH~ and C1+ fragments in coincidence experiments as shown in Fig. 2. From this figure, it is clear that the appearance threshold for the IA' is 32.70 eV, while that of the 3A 2 state is governed by the double ionization energy, i.e. 30.69 eV. The first value is in good agreement with the experimental one at 32.5 + 0.5 eV (Table 1), which indicates that the singlet
state fragmentation is observed. Thus, we conclude that the triplet state must follow another dissociation channel. By using the theoretical energy values and ZPE corrections for CH~ and C1÷, it is also possible to calculate the KER of the dissociation reactions. The obtained results are 6.38 eV for Eq. (2) and 5.96 eV for Eq. (3). Both values are in the experimental range (5.4 ___ 1.0 eV) and it is not possible to discriminate between the two processes. After bombarding neutral CH3C1 with 1 MeV atomic ions, Monce and co-workers [5,6] observed the CH~/C1 ÷ fragments, with a KER of 6.6 +- 0.1 eV. This result is close to the value obtained for the ~A' ground state of CH3C12÷ (6.38 eV). 4.2. CH3C12+ ---* CH2C1 + + H +
This dissociation pathway is the second most important one observed in the PEPIPICO experiments of Ruhl and co-workers [7] with 13% of the fragment products and a measured KER of 5.2 ___ 2.0 eV (Table 1). To produce the CH2CI÷/ H ÷ pair, we calculated the transition state geometries obtained by removing one H atom from the equilibrium geometries of the ~A' and 3A 2 states of CH3C12÷. This led to the 5 and 6
34
32.70
32
I A' 31.64-
31.43
3A 2 3 0 . 6 9 ~
3O
/
29.37
~
29.74
28
r~
26 24.73
24 CH3CI++
CH3+ + CI +
CH2+ + C I +
22 Fig. 2. Energy d i a g r a m for CH3C12÷ ~ CH~ + CI ÷ dissociation.
+1t
D. Duflot et aL/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
223
36.71 36
34.2~-'~ HCCI+e+H + H
34.28 34
~t3~ 32.44
32
I A ' 31.64 3A 2 30.69
188/
/
\
30
\
\
29.74 \29.01 CCI+ + H+ + H2
t~
~33.31
CCI+ + H+ + H + H
CH2+ + CI+ + H
28
26
CH3 C I+ + 24
Fig. 3. Energy diagram for CH2C12+production and dissociations.
Eq. (4) corresponds to the direct dissociation of CH3C12+(IA') in the three CCI+/H2/H + fragments (three-body process, discussed in Section 5). The CH2C12+ dication produced in Eq. (5) is not found in the experiments of ref. [7] but is present as a very weak signal in ref. [11] at 32.0 -+- 1.0 eV (Table 1). This value is in very good agreement with the calculated one at 32.25 eV. The predicted KER would then be very low (0.37 eV). It is probable that most of the CH2C12+ dication
structures shown in Fig. 1, respectively. Mulliken population analysis [31] showed that the reactions were in fact (see Fig. 3): CH3C12 + (1A') ---* CC1 ÷ - - H2 - - H + (1A') ---*CCI+(1E+)+H2(I~:g)+H +
3 2 . 4 4 e V (4)
CH3CI 2+ (3A2) ---. CH2C1 - _ H 2÷ (3A2) ---~ CH2C12+ (2Bl) + H(2S)
32.25 eV
(5)
34
32
I A ' 31.643A230.69-
30
30.79 -- ~
~
o~
30.71 29.76
\
H/-H-'~I+ +H+ +H
/,.o,
28
27.57
r~ 26
~_~ 26.09
24
CH3CI+ +
CH2CIH+ +
CH2CI+ + H+
22
Fig. 4. Energy diagram for CH3C12+~ CH2CI+ + H + dissociation.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
224
dissociates spontaneously. Thus, it is clear that the CH2CI+/H + pair cannot be obtained by eqns (4) and (5). Another possibility is to break the H C1 bond of the methylechloronium dication CHzC1H 2+ (structure 8a in Fig. 1) after a preliminary isomerization of CH3C12+ (structure 7 in Fig. 1). The isomerization reaction has been studied in our previous paper [1] for the 1A' state only, since the triplet transition state was found to dissociate along the C-CI coordinate. For this reason, the non-planar 3A" state of CH2C1H 2+ (structure 8b in Fig. 1), which lies 31.17 eV above the energy reference was not considered in the present study. The resulting dissociation pathway as displayed in Fig. 4 is the following: CH3C12+ (1A ') ---* CH~- - - H -
- C I + (IA)
(6) ---* CHzC1H 2+(IA')
30.79 eV
in the PEPICO ones [11] as a very weak signal. The appearance threshold is difficult to determine and is at least 32.0 eV (Table 1). It is important to notice that coincidence experiments do not detect neutral fragments. Hence, the HCCI 2+ ion could correspond to a three-body dissociation with production of two separate H atoms. However, the geometry of the ~A' state of CH3C12+ (structure 3a in Fig. 1) suggests that this state is a weakly bound complex of H 2 with HCC12+. Thus it should be easier to produce the H2 molecule, as revealed by our calculations: the structure of the corresponding transition state (10) is shown in Fig. 1 and the dissociation pathway in Fig. 5, which shows that the height of the energy barrier is only 0.6 eV: CH3C12+ (JA ,) ---. HCC12+ _ - Hz(1A ') HCClZ+(I~2+)+H2(II2g)
29.97 eV
(8)
CH2C1H 2+ (1A') ---*CH2C1 - - H 2+ (1A')
('7) ---* CH2CI+(1AI)+H +
29.76 eV
Eq. (7) has already been studied theoretically by Yates and co-workers [2] at MP3/6-31G**// RHF/6-31G* level. The geometry of their transition state is shown in parentheses in Fig. 1 (structure 9). The agreement between the two calculations is rather good. The isomerization energy of 2.19 eV is close to the previous value of 2.08 eV [2]. The fragments are 1.48 eV below CH2CIH 2+, which is very similar to the result of ref. [2] (1.53 eV). Finally, it is clear from Fig. 4 that the theoretical appearance threshold is given by the vertical ionization energy of the singlet state, i.e. 31.64 eV. This result is slightly higher than the experimental threshold at 31.0 _+ 0.5 eV (Table 1). The calculated KER is 5.55 eV, in good agreement with the experimental value of 5.2 _+ 2.0 eV (Table 1). 4.3. C H 3 C l 2+ ~
H C C l 2+ + H 2
The HCC1 e+ dication is not detected in the PEPIPICO experiments of ref. [7] but is present
The theoretical appearance threshold is governed by the ionization energy of the singlet state, i.e. 31.64 eV, in good agreement with the experimental value at 32.0 eV. 4.4. CH3Cl 2+ ---* H C C l + + H~
The HCCI+/H~ ion pair is the weakest twobody process observed in PEPIPICO experiments [7], with 2% of the final fragments and a KER of 2.8 _+ 1.0 eV (Table 1). From the theoretical point of view, these fragments may be obtained by removing two H atoms from the 3A 2 C3vsymmetric state of CH3C12+ (Fig. 5): CHC12+ (3A2) ~ HCC1 + - - H~- (3A") ---~HCCI+ (2A') + H~- (2~g)
33.65 eV
(9)
The geometry of the corresponding 3A" transition state is shown in Fig. 1 (structure 11). However, the calculation of the harmonic vibrational frequencies gave three imaginary modes, the largest one (i1358 cm -t) corresponding to the movement of the one H atom out of the symmetry plane of the molecule. Curiously, the CHz/CH3 symmetric stretching mode, which is the reaction coordinate, is very low (i139cm-J). The i195cm -I
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
225
33.65 33.65
34
32"43
32
IA'
31.64-H 2 + + CCI + + H
3A 230.69
1 H + + HCCI + + H
30 ¢0
1 28
H 2 + CCI + + H +
28.13
¢d
H2+ + HCCI ÷ 26
24
CH 3 C I+ + 22 Fig. 5. E n e r g y d i a g r a m f o r C H 3 C I 2+ ---* H C C I -
mode is the antisymmetric stretching. This indicates that the true transition has no symmetry at all. In order to find its geometry, we performed test calculations with a configuration space reduced to quadri-excitations. Unfortunately, we were not able to locate this particular point of the triplet potential energy surface. Thus, we can only say that the appearance threshold for this pair is below 33.65 eV and the KER is below 5.52 eV. These values are consistent with the experimental ones. 4.5. C H 3 C I 2+ ---* CCl + + H~
The production of H~ is the most surprising process occurring in the fragmentation of CH3X 2+ compounds [7,10]. Indeed, it requires the breaking of the three CH bonds and the formation of new H - H bonds. In the case of CH3C1 z+, it represents 6% of the final products [7], which is rather important, with a KER of 3.4 + 1.0 eV and a very low appearance threshold [ 11] of 30.0 + 0.5 eV (Table 1). The formation mechanism of H~ is unknown. In the case of methylamine [10], experiments with deuterated species proved that the H~ ion came from the methyl group rather than the amine one. Thissen [10] proposed that the formation of the dication leads to the formation of an
+ H2 p r o d u c t i o n a n d d i s s o c i a t i o n s .
H2 molecule weakly bound to the carbon atom. The dissociation of this structure would give the H2PrlCNH~÷ fragment pair and, owing to the large protonic affinity of H 2 (4 eV), the possible formation of H~ and CNH~. Thus, such a mechanism might also occur in the case of CH3CI 2+, since, as already seen, the ground 1A' state is a weakly bound complex of H 2 and HCC12+. In order to test this hypothesis and explain the presence of H + 3, it is necessary to determine the geometry of a possible transition state between CH3C12+ and CCI+H~. This is a difficult task since this geometry cannot be simply guessed, contrarily to other mechanisms studied in the present work. The usual method consists of examining the shape of the normal modes of the molecule. Because of a too large computational cost, we limited the search to the ~A' state of CH3C12÷, for which we made SCF test calculations before the CASSCF ones. Indeed, the 3A2 state is not stable at SCF level [1]. The CASSCF harmonic frequencies of CH3C12+(1A ') are very similar to the SCF ones (see ref. [1]): the two lowest ones (377 and 800 cm -~ correspond to the rotation of the weakly bound HzH3 group, leading to the (12) Cssymmetric planar geometry shown in Fig. 1.
226
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
Obviously, this structure has one imaginary frequency (i326 cm -~) which is the out-of-plane rotation of the H2H3 group. By increasing the H2CC1 angle, we obtained the (13) C2v-S ymmetric planar structure shown in Fig. 1, which also has one imaginary frequency (i1356cm -~) corresponding to the in-plane HzCC1 bending mode. By increasing order, the next frequencies of CH3C12+(1A') are the following: the 867 cm -I mode is the movement of H1 out of the symmetry plane of CH3C12+, the 969 cm -1 mode is the CH3CH 4 symmetric stretch with its anti-symmetric counterpart at 1121 cm-1; the 1060 cm -~ frequency corresponds to the CC1 stretch. The 1409 cm -~ mode seems more interesting, since it corresponds to a simultaneous increase of the three HCC1 angles. However, SCF calculations failed to locate any stable geometry. Finally, the 2953 cm -1 mode is the CH1 stretching and the 3091 cm -1 mode is the H2CH3 bending mode. Thus, it does not seem possible to produce the H~ ion by following the normal modes of the 1A' state of CH3C12+. However, while exploring the potential energy surface of this state, we found that the linear ~E+ structure depicted in Fig. 1 (structure 14) was stable at both SCF and CASSCF levels. Analysis of its vibrational
frequencies shows that this structure is a saddle point with a doubly degenerated imaginary mode (i269 cm -~) corresponding to the rotation of the H2H 3 group. The most surprising result is that it lies at 29.75 eV, i.e. 0.38 eV above the equilibrium geometry. It is then possible to produce the H~/CC1+ pair by increasing the CH1 bond length (structure 15 in Fig. 1): CH3C12+ (1]~+) ~ CC1 + _ - H f (]~ +) (10) --,CCI+(I~+)+H~-(1A1')
Of course, the structure 15 is not, strictly speaking, a transition state, but it appears that it is rather easy to obtain the H~ ion from CH3C12+(1A'). Tentative calculations to find a true transition state by rotating the H2H3 group in 14 failed or led to 13. The whole dissociation pathway is displayed in Fig. 6. From this figure, the appearance threshold is governed by the singlet ionization energy at 31.64 eV, well above the experimental one at 30.0 -+ 0.5 eV. Similarly, the predicted KER is 5.55 eV which is larger than the experimental value of 3.4 _+ 1.0 eV. Because of these large discrepancies, we suggest that the presence of the H~ ion is not due to a direct dissociation of CH3C12+ as given in Eq. (10) but rather to the
34
32
IA' 31.64-
30.74
3A 2 3 0 . 6 9 30
oj
29.77 eV
+ H2+ + H
29.77 ~ 2 9 . 7 5 29.37
28
r~
26 26.09
24 CH3CI++ H2-HCCI++
CCI+ + 113+
22 Fig. 6. Energy diagram for CH3CI2÷--* CC1÷ + H~ dissociation.
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
ionization of a Rydberg excited state of the monocation. This would explain the fact that the appearance threshold of the CCI+/H~ pair is lower than the double ionization energy• Such a mechanism was observed for the first time without ambiguity in the case of H 20 2+fragmentation [32]. More recently, the analysis of the KER distributions (KERD) showed a significant contribution of the indirect processes in the case of CO, 02 and NO dications at an excitation energy of 40.8 eV, owing to the autoionization of the oxygen atom [33]. Concerning CH3C12+, another possibility is the implication of the triplet state, which was not considered here, on condition that its ionization threshold is close to the calculated value of 30.69 eV, rather than the experimental one. 4.6. C H 3 C I 2+ ~
[2] in parentheses. It is remarkable that the C-C1 distance is more than 0.5 A greater in our calculation. The isomerization barriers are also different: 2.18 eV instead of 2.48 eV given in ref. [2]. The two fragments are 1.94 eV below CH2C1H 2+ (1.66 eV in ref. [2]). The whole dissociation pathway is shown in Fig. 7, from which we deduce a theoretical appearance threshold of the ion pair of 31.64 eV in good agreement with the experimental value of 32.0 ± 1.0 eV (Table 1). The predicted KER is 6.00 eV. However, this pair is not present in the PEPIPICO spectrum of Ruhl and co-workers [7].
5. Three-body processes Interpretation of PEPIPICO spectra for threebody processes is much more difficult than for two-body processes [34]. In our previous study [26,35] of the fragmentation of CzH~+, we explained that ab initio calculations alone do not allow determination of the precise mechanisms of the fragmentations processes, because it is necessary to add the theoretical energy of the products to the measured KER to obtain an estimation of the appearance threshold. In the case of
CH~ + HCI +
The CH~/HC1 + pair can be obtained through dissociation of the CHzC1H 2+ after preliminary isomerization of CH3C12+ via Eq. (6): CHzC1H2+ (IA ') ----*CH2 - - C1H2 + (IA ')
(11) --*CHf(2AI)+HCI+(2H)
29.75 eV
The geometry of the transition state (16) is shown in Fig. 1, with the values of Yates and co-workers 34
•
32 ~" 3O
IA' 31.64-
~
\
/
CI+( I
~
~
31"43D)+H(2S)
~
[
\
I 29.41/
~~ ~
It-
[[ 29.74
28 27.57 26
25.63 (2H)
24 CH3CI++
227
CH2CIH++
CH2 + + HCI + CH2+ + CI + + H
22
Fig. 7. Energy diagram for CH3C12+~ CH~ + HCI÷ dissociation.
228
D. Duflot et aL/International Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
CH3C12+, the situation is even more complex, because of the large number of possible dissociation channels and the low precision of the KER measurements (Table 1). 5.1. C H 3 C I 2+ ---* C H ~ + C I + + H
This dissociation pathway is the most important three-body process observed in ref. [7], since the CH~/C1 + ion pair represents 9% of the observed products with a KER of 4.3 _+ 1.0 eV (Table 1). From the coincidence peak slope, it was deduced that the H atom was formed by decay of CH~. From the theoretical point of view, these fragments can be obtained in different ways: C H f (IA1 ') +CI+(1D) ---. CH~- (1A1 ' ) (12) +CI+(1D)+H(2S)
CH~-(1A 1') +C1 + (3p) ----~C H f (IA'l)
(13)
+ C1 + (3p) + H(2S)
5.2. CH3C12+ ---* H C C I + + H + H +
The HCCl+/H + ion pair represents 4% of the final products in ref. [7] with a KER of 3.6 2.0 eV (Table 1). The slope of the experimental peak does not allow identification of the mechanism of the fragmentations. There are only two possibilities (Figs. 4 and 5): CHzCI+(1A1) + H + ---, HCCI+ (2A') +H(2S) +H+ (16) HCC1 + (2A') + H~- (2~g) ---, HCC1 + (2A')
(17) + H(2S) + H + The appearance threshold is predicted to be 34.3 _+ 2.0eV, in good agreement with the measured one at 35.0 -+ 0.5 eV (Table 1). However, it is not possible to decide which of the two processes occurs in the fragmentation. Quite unexpectedly, the CH 2C12+(2 B 1) dication dissociates in the following way (Fig. 3): CH2 C12+ (2B l) + H( 2S) ---*HCC12 ÷ (1 ~ + )
CH~- (2A1) + HC1 + (21-I) ---* CH~- (2A1) (14) +CI+ (1D) +H(2S)
(18) +H(2S)+H(2S)+H(2S)
34.03eV
CH2C12 + (2B 1) + H(2S) ~ CH~- - - C1 + (2B l ) 5.3. C H 3 C I 2+ ---* H + + C C I + + H 2
+ H(2S) --+ CH2(2A1) + C1 + (ID) + H(2S)
(15)
Eqs. (12) and (13) are depicted in Fig. 2. We can predict an appearance threshold of about 34.0 _+ 1.0 eV for the triplet state and 35.7 _+ 1.0 eV for the singlet one. Both values are consistent with the experimental one at 35-36 eV (Table 1). Eq. (14) is shown in Fig. 7 for the singlet state, with a theoretical threshold of 35.7 _+ 1.0 eV. Finally, Eq. (15), which is a 'deferred charge separation' [32], presents two energy barriers (Fig. 6), the higher one at 34.03 eV, leading to a predicted threshold of 38.3 -+ 1.0 eV, which is obviously too high. The calculations cannot discriminate between the eqns (12)-(14) processes, but the last one Eq. (15) is excluded by experiments [7].
The CCl+/H + ion pair represents 3% of the fragments observed in ref. [7] with a KER of 3.6 _+ 2.0eV (Table 1). Its origin remained ambiguous. Assuming that the two H atoms form a molecule, the first possible dissociation pathway is the 'instantaneous explosion' [32] already shown in Eq. (4) of Section 4.2 (Fig. 3). However, the predicted appearance threshold is 32.44eV, which is below the measured one at 35.5 -+ 0.5 eV (Table 1). Other possible channels are the following (Figs. 4 and 5, respectively): CH2CI+ (IAl) + H + ---, CCI+ (lp. +) (19) +H + +H2(l~;)
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
HCC12+ (1~ + ) + H2(l~g) ---. CCI+ (1 ~ + ) (20) + H + +Hz(lZg) For both processes, the predicted appearance threshold is 32.6 + 2.0 eV, which is below the experimental one at 35.0 _+ 0.5 eV (Table 1). It is also possible to produce two separate H atoms, after production of HCC12+ via Eq. (18), in the following manner (Fig. 3): HCCI2+ (lZ+) + H(2S) +H(2S) ---. H + +CCI+(I~+)+H(zS)+H(2S)
229
3.6 --_ 2.0 eV (Table 1). The coincidence peak slope indicates without ambiguity that the C1 atom is formed by decay of HC1 + into H + + CI. Since the 21-Iground state of HC1 ÷, which correlates to the ~A' and 3A" lowest states of CH2C1H 2+, dissociates into H + C1+ as shown in Eq. (14), this ion pair must come from an excited state of HC1 ÷. A possible candidate is the first one, i.e the 2~:+ state. The dissociation pathway is the following, after isomerization to produce CH2C1H 2+ (Fig. 7): C H f (2AI) + HC1 + (2]S+ ) -.--,CH~- (2A1)
36.71eV
(24) (21)
Thus, we have the very remarkable series of deferred charge separations for the 3A2 state of CH3C12+: CH3C12+ ~ C H z C I 2+ --, HCCI 2+ ~ H + + CC1 +. For Eq. (21), the predicted appearance threshold is 36.71 eV, which is above the experimental one, although the calculated KER of 3.4 eV is close to the measured one.
+ H + + CI(2p) In this case, the final products have an energy of 30.99 eV, and the predicted appearance threshold is about 34.6 _+ 2.0eV, which is below the experimental one at 37.5 + 1.0eV (Table 1). Because of the large uncertainties, it is not possible to give more precise information on the origin of this ion pair.
5.4. CH3CI e+ ---, H + CCI ÷ + H~
The CCI+/H~ ion pair is not detected in the PEPIPICO experiments of ref. [7] but is present in the PEPICO ones [11] with an appearance threshold of 36.0 _ 0.5 eV (Table 1). The possible dissociation pathways are the following (Figs. 5 and 6): HCC1 + (2A') + H~- (2~g) --, H(2S) (22) +CCl+(l~+) +H2(2~g ) CCl+ (J £ + ) + H~-(IAl ') ---, H(2S) (23) The calculated energy of the final fragments (30.74 eV) combined with the measured appearance threshold gives a predicted KER of 5.26 _+ 0.5 eV. 5.5. CH3CI 2+ ---, CH~ + H + + CI
The CH~/H + ion pair represents 2% of the fragments observed in ref. [7] with a KER of
6. Conclusion In this paper, we have used large scale ab initio calculations to study the fragmentation processes of the chloromethane dication. This study was made difficult for several reasons. Except the CH~/CI+ and CH2CI+/H ÷ dissociation pathways representing 74% of the total intensity, all other channels contribute individually to less than 10%, especially the three-body processes. All two-body dissociations occur in a very small energy range centered around 31 eV and the uncertainties of the available experiments do not allow a good discrimination between the processes. Nevertheless, our calculations have explained most of the two-body thresholds by a direct dissociation of the lowest singlet and triplet states of CH3C12+, taking into account the experimental uncertainties. It appears that the production of
230
D. Duflot et al./lnternational Journal of Mass Spectrometry and Ion Processes 171 (1997) 215-230
the H~ ion is due to an indirect process, involving the ionization of a Rydberg excited state of the monocation. For three-body processes, the large uncertainty on the measured KER and the number of possible channels does not allow us to determine precisely the fragmentation pathway in most cases. More precise experiments, combined with dynamical calculations (using RRKM theory for example) are needed to understand in detail the fragmentation of CH3C12+. Another problem is the calculation of the double ionization threshold of molecules. From the computational point of view, the present work shows that the study of the CH3-X 2+ dication family represents a challenge for ab initio calculations. The marginal stability of the species requires the use of extended basis sets combined with correlated methods, as already shown [1]. The use of the CASSCF method is well suited for studying dissociation processes. However, as illustrated in the present paper, the computational cost does not allow us to explore fully the potential energy surfaces of the molecule. It does not seem possible to apply this method to larger C H 3 - X 2+ dications, such as methylamine or methanol [7]. The problem is then to find a good compromise between accuracy and computational time.
Acknowledgements We are very grateful to Dr Roland Thissen for providing us with his unpublished experimental results and for reading carefully this manuscript. The Laboratoire de Dynamique MolEculaire et Photonique is 'Unite de Recherche AssociEe au CNRS'. The Centre d'Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Fonds EuropEen de D6veloppement Economique des REgions. The authors acknowledge the CNRS (Mathematical and Physical Science Department) and the MinistEre charge de la Recherche for a generous allocation of computer time on the IBM RISC 6000 at IDRIS (Orsay).
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